Refactor imports in MainActivity to include specific GenAI classes

Author: vraspar

mobile/examples/phi-3/android/app/src/main/java/ai/onnxruntime/genai/demo/MainActivity.java

@@ -26,7 +26,9 @@
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.function.Consumer;
 
-import ai.onnxruntime.genai.*;
+import ai.onnxruntime.genai.SimpleGenAI;
+import ai.onnxruntime.genai.GenAIException;
+import ai.onnxruntime.genai.GeneratorParams;
 
 public class MainActivity extends AppCompatActivity implements Consumer<String> {